The present invention relates generally to batteries and more particularly to an electrochemical battery which utilizes a highly reactive "consumable" anode material such as the alkali metals.
A typical electrochemical battery includes an outer casing and electrically insulated terminals which together define a sealed interior chamber. It also typically includes an arrangement of chemically interacting components located within the chamber for producing a voltage drop of a characteristic volume across the terminals. A common arrangement of this type is made up of an anode, an electrically insulated and spaced apart cathode and an electrolyte solution which fills the chamber and surrounds the various other components making up the arrangement.
To meet substantial demands for higher performance batteries, substantial work has been done with cell chemistries using an alkali metal anode and particularly lithium. The cathode and electrolyte material consisting of a solvent and solute vary. Indeed, the literature is replete with examples of alkali metal anode cells with different cathodes and electrolytes. The electrical characteristics of these cells such as energy per unit volume, called energy density; cell voltage; and internal impedance vary greatly.
Among all the known combinations of lithium anodes with different cathodes and electrolytes, those believed to have among the highest energy density and current delivery capability use certain inorganic liquids as the active cathode depolarizer. This type of cell chemistry is commonly referred to as liquid cathode.
The use of a liquid as an active cathode depolarizer is a radical departure from conventional cell technology. Until recently, it was generally believed that the active cathode depolarizer could never directly contact the anode. However, it has recently been discovered that certain active cathode materials do not react chemically to any appreciable extent with an active anode metal at the interface between the metal and the cathode material, thereby allowing the cathode material to contact the anode directly.
A major step forward in the development of liquid cathode cells was the discovery of a class of inorganic materials, generally called oxyhalides, that are liquids at room temperature. These materials perform the function of active cathode depolarizers. Additionally, they may also be used as the electrolyte solvent. Liquid cathode cells using oxyhalides are described in U.S. Pat. No. 3,926,699 issued to Auborn on Dec. 16, 1975 and in British Pat. No. 1,409,307 issued to Blomgren, et al. on Oct. 18, 1975. At least one of the oxyhalides, thionyl chloride (SOCL.sub.2), in addition to having the general characteristics described above, also provides substantial additional energy density and current delivery capability.
As described in the Auborn and Blomgren patents, the anode is lithium metal or alloys of lithium and the electrolyte solution is an ionically conductive solute dissolved in a solvent that is also an active cathode depolarizer. Regarding this latter constituent, specifically the electrolyte solution (or "electrolyte" generally), in the various preferred embodiments of the present invention to be discussed hereinafter, this solution including its solute and solvent, which also acts as the active cathode polarizer (liquid cathode), comprises part of the chemically interacting constituents referred to above.
The solute may be a simple or double salt which will produce an ionically conductive solution when dissolved in the solvent. Preferred solutes are complexes of inorganic or organic Lewis acids and inorganic ionizable salts. The requirements for utility are that the salt, whether simple or complex, be compatible with the solvent being employed and that it yield a a solution which is ionically conductive. According to the Lewis or electronic concept of acids and bases, many substances which contain no active hydrogen can act as acids or acceptors or electron doublets. In U.S. Pat. No. 3,542,602 it is suggested that the complex or double salt formed between a Lewis acid and an ionizable salt yields an entry which is more stable than either of the components alone.
Typical Lewis acids suitable for use in the present invention include aluminum chloride, antimony pentachloride, zirconium tetrachloride, phosphorus pentachloride, boron fluoride, boron chloride and boron bromide.
Ionizable salts useful in combination with the Lewis acids include lithium fluoride, lithium chloride, lithium bromide, lithium solfide, sodium fluoride, sodium chloride, sodium bromide, potassium fluoride, potassium chloride and potassium bromide.
The double salts formed by a Lewis acid and an inorganic ionizable salt may be used as such or the individual components may be added to the solvent separately to form the salt. One such double salt, for example, is that formed by the combination of aluminum chloride and lithium chloride to yield lithium aluminum tetrachloride.
In addition to an anode, active cathode depolarizer (liquid cathode) and ionically conductive electrolyte, these cells require a current collector or cathode collector which is of particular interest to a number of aspects of the present invention.
Applicant has found that there are a number of factors relating to cathode current collector material selection and assembly procedures that dramatically affect the performance of these cells. In particular applicant has found that the energy density, power density and attitude sensitivity of these type cells are strongly affected by the cathode current collector.
Some of the problems that applicant's invention addresses are.
First, the anode in these cells in consumable. That is, an anode constructed of, for example, magnesium or a more highly reactive material such as lithium or sodium gradually decreases in size, at least to a limited extent, as it dissolves during the discharge of the battery.
While the utilization of a consumable anode has its advantages, it also has disadvantages. Specifically, as the anode is consumed and therefore gradually decreases in size, the spacing between the anode and cathode collector increases. This, in turn, reduces the overall efficiency of the various components to interact with one another for producing the desired voltage drop. Moreover, it should be apparent that as the anode gradually loses volume, the effective volume within the chamber can increase. This, in turn, could create gas pockets or voids within the electrolytic solution surrounding the anode and cathode collector which could reduce the efficiency of the battery if these voids locate themselves in proximity to the anode.
Secondly, it has been found that the particular attitude, that is, the relative position of a battery of the general type described above may affect its efficiency. More specifically, it has been found that a battery of this type, if maintained in an upright position (top side up), generally displays a longer life than if it were maintained in a different position, for example on edge (90.degree. from its upright position) or upside down (180.degree.). It is believed, at least by some, that this is the result of the presence of gas pockets or voids within the battery chamber, as described above. In any event, it is a very real phenomenon and can be a serious drawback to electrochemical cells or batteries of the type described.
One solution for overcoming at least some of the disadvantages described above is disclosed in U.S. Pat. No. 3,985,573 (Johnson et al). This patent discloses an electrochemical cell and specifically one which utilizes a highly reactive anode material such as lithium, sodium, and the like, in conjunction with high energy liquid cathode materials and nonaqueous electrolytes. Moreover, this patent recognizes some of the disadvantages in utilizing a consumable anode such as the decrease in anode volume during discharge which tends to increase the distance between the anode and other components, thereby decreasing the contact between the anode and these other components.
The Johnson et al patent also discloses two prior patents, specifically U.S. Pat. Nos. 3,809,580 and 3,796,606. According to Johnson et al, the '580 patent overcomes the disadvantage just described by providing a roll or coiled electrode assembly, a so called "jellyroll" construction, to insure good contact between the components of the cell during discharge. The '606 patent, according to Johnson et al, discloses a cylindrical electrochemical cell utilizing, among other components, a porous separator and an elastically deformable current collector having a split cylindrical shape in which the elasticity of the current collector enables it to maintain bias contact with the negative electrode at all times, notwithstanding alteration in electrode volumes during discharge of the cell. On the other hand, Johnson et al solves this problem by utilizing, among other components, an elastically deformable carbonaceous cathode collector in the form of a slotted annular bobbin.
While the various solutions just discussed may well maintain a continuous bias on a consumable anode as the latter decreases in size or volume, they do include a number of their own disadvantages. For example, in at least one case, specifically Johnson et al, the solution described requires additional foreign matter within the cell not otherwise required for producing a voltage drop. And, as will be described later, the addition of foreign matter reduces battery efficiency. Moreover, in the '580 and '606 patents, as well as in Johnson et al, the inner components seem to be relatively complicated in design. It should be quite apparent that the addition of foreign matter within the battery and the utilization of complicated components can be relatively expensive. In addition, the use of foreign and/or complicated internal components may result in less available space for voltage producing components, thereby decreasing overall efficiency or it may result in contamination within the cell.
Johnson et al is also found to be lacking in that this patent does not even address itself to the previously discussed problems resulting from the presence of gas pockets or voids within the cell. The utilization of an elastically deformable carbonaceous cathode collector, as described in Johnson et al or, for that matter, the utilization of a jellyroll construction or elastically deformable current collector as recited in Johnson et al does not eliminate or minimize the presence of these voids.
In U.S. Pat. No. 3,907,593 (Marincic), dated Sept. 23, 1975, an electrochemical cell is disclosed utilizing a cathode material which is recited as carbon such as graphite, carbon black, or acetylene black. While carbon is a porous and relatively high surface area material, particularly acetylene black which is highly desirable, the cathode structure disclosed in this patent does not consist essentially of this material but rather includes what is referred to as a screen, presumably to support the cathode material. Unfortunately, this screen is relatively inert with regard to the production of cell voltage and, moreover, it takes up precious space which could be utilized by an "active" constituent. In addition, as recited in the EXAMPLES set forth in the Marincic patent, the cathode material, apart from the screen, is not pure but rather includes 86% acetylene black, 10% graphite and 5% polytetrafluoroethylene binder. This binder, like the screen, does not add to the production of cell voltage but rather takes up precious space which, again, could be utilized by an active constituent. Moreover, it tends to coat the carbon, reducing its effectiveness.
As will be seen hereinafter, the present invention is directed to a battery and particularly to an electrochemical battery which overcomes the aforedescribed disadvantages, in an uncomplicated, economical and reliable manner.